JVM principle

One 、 Class loading

1、 Start class loader
Extend the classloader
Apply the class loader ： User defined class loading

2、 Parents delegate ： For example, when user-defined classes are loaded, they will ask their superiors , Until the class loader is started to load , If the startup class loader cannot complete the loading task , It is still left to the first requested class loading ( Here is the application class loader ). The advantage of this is , After the same class is loaded by different class loaders , Not the same class , It will cause confusion in the management of these classes , And if it is all started by the class loader java.lang.Object Classes are generated in the same Object The next is the same class , Easy to manage .

Two 、gc Garbage collection

( Reference resources https://www.sohu.com/a/235928510_100123073)

Three perspectives to understand gc：
1jvm How to determine which objects should be recycled ？
Two classical algorithms for whether objects will be recycled ：
Reference counting ( Not quoted , If it is a circular reference , Can't solve this problem , such as a,b,a Call in b,b call a)
* Reachability analysis algorithm （ The virtual machine uses a directed graph , The program treats all references as a graph , Through a series called GC Roots As the starting point , Start with these nodes and drill down , The path a search takes is called a reference chain . When an object arrives GC Roots There's no chain of references connected ( It's from GC Roots It's impossible to reach this object ) when , Then prove that this object is not available .　 Pictured ：）

2jvm When will garbage collection be performed ？
answer ：1 Will be in cpu Automatically recycle when free 　2 After the heap memory is full 　3 Active call System.gc() Try recycling after

3jvm How to clear garbage objects ？
The algorithm has four more ： Mark - Clear algorithm , Copy algorithm , Mark - Sorting algorithm , Generational collection algorithm ( The new generation , Old age , Forever (1.8 And later called metadata area ))

1、System.gc() and Runtime.gc() What will be done ？

These two methods are used to prompt JVM To recycle . however , Whether garbage collection starts immediately or is delayed depends on JVM Of .

2、 But there is one kind. JNI(Java Native Interface) call non-Java Program （C or C++）,finalize() My job is to recycle this part of memory .
3、. If a reference to an object is set to null, Whether the garbage collector will immediately release the memory occupied by objects ？
Can't , In the next garbage collection cycle , This object will be recyclable .